The present invention relates to a tilt angle adjustment method for adjusting a tilt angle of an objective lens and an optical information recording/reproducing apparatus in which the objective lens is mounted, and particularly to a tilt angle adjustment method suitable for adjusting a tilt angle of an objective lens for a multilayer high recording density optical disc and an optical information recording/reproducing apparatus in which such an objective lens is mounted.
There exist various standards of optical discs, differing in recording density, protective layer thickness, the number of recording layers, and etc. Various types of optical information recording/reproducing apparatuses suitable for recording information to and/or reproducing information from the optical discs based on the various standards have also been proposed and in practical use. Japanese Patent Provisional Publication No. 2004-171771A (hereafter, referred to as JP2004-171771A) discloses an example of an optical information recording/reproducing apparatus suitable for information recording or information reproducing for a multilayer DVD (Digital Versatile Disc). The optical information recording/reproducing apparatus disclosed in JP2004-171771A is configured to suitably correct the spherical aberration caused by the difference between recording layer thicknesses (i.e., thicknesses of protective layers protecting respective recording surfaces of layers). Incidentally, in this specification, the “optical information recording/reproducing apparatuses” include apparatuses for both information reproducing and information recording, apparatuses exclusively for information reproducing, and apparatuses exclusively for information recording.
Recently, further enhancement of the recording density of a recording medium is advancing, and a high recording density optical disc having a further higher recording density than DVD has put in practical use. One of such a high recording density optical disc is BD (Blu-ray Disc). Study for multilayer technology for such a high recording density optical disc has also been made, and standardization of the multilayer high recording density optical disc is advancing.
For information recording or information reproducing for the high recording density optical disc, a laser beam having a short wavelength and an objective lens having a higher NA than that for DVD are used. Therefore, when the high recording density optical disc is used, sensitivity of a coma with respect to decentering of an objective lens and tilting of an optical disc becomes high. In general, a decentering coma caused by decentering of an objective lens and an off-axis coma caused by inclination of an incident light beam due to, for example, an installation error of a LD (Laser Diode), or individual differences or an installation error of a reflection prism can be cancelled by adjusting a tilt angle of the objective lens so as to cause a coma in an inverse direction.